I checked the Setup.log files (In Setup and Data\log directories) for a few days.
It seems that in the Setup.log files, only lines are added when I'm administrator, at least so far regarding the updates.
Both files end with "Stopped: 14.03.2007, 21:52:31", which was the time of the successful update as administrator.
Further on, the following files where last time changed after successful update (when I was adminstrator).
Avast\Setup\part-vps-72302.vpu
Avast\Setup\vps-72300.vpu
Avast\Setup\vpsm-72302.vpu
Avast\Setup\setup.log
Avast\DATA\400.vps
Avast\DATA\log\Setup.log (has already Full Control for Everyone")
Giving them "Full Control" rights like administrators isn't a solution as well.
The file "Avast\DATA\log\Warning.log" has many errors for failed updates as follows:
<date & time> 1173982026 <my username> 3500 Function setifaceUpdatePackages() has failed. Return code is 0x00000426, dwRes is 00000000.
<date & time> 1173982422 <my username> 1092 Function setifaceUpdatePackages() has failed. Return code is 0x00000426, dwRes is 72FE02C0.
PS: Only numbers after "Return Code" are mostly the same.
I will dig some more into the "setifaceUpdatePackages()" message on this forum, but so far not much useful found